In cordless digital communication systems which are based on the Bluetooth, DECT, WDCT standard or a similar standard, traditional signal processing methods are used for demodulating the received signal and for the signal detection at the receiver end. Receiver designs are known in which the intermediate-frequency signal is converted into the digital domain with the aid of an analog/digital converter and the signal detection is implemented with the aid of methods of digital signal processing. Using such methods, a high-quality signal detection can be achieved but it is disadvantageous that an elaborate analog/digital converter is needed. Such a method, which is frequently used, is based on the so-called limiter-discriminator demodulator in which, after hard limiting of the band-pass signal which, as a rule, is complex, the received angle-modulated signal is demodulated, e.g. by means of an analog coincidence demodulator.
In the printed document DE 102 14 581.4, an intermediate-frequency receiver is described which uses a zero-crossing detector for the signal detection. The zero-crossing detector measures the time intervals between the zero crossings of a received intermediate-frequency signal and determines the transmitted data symbols from the zero-crossing information. For this purpose, the sequence of zero-crossing intervals output by the zero-crossing detector is stored in digital form in a shift register chain and compared with previously stored zero-crossing reference sequences in a classification unit. A city block metric is proposed for measuring the distance between the sequences measured and the sequences stored. The previously stored zero-crossing reference sequence which has the smallest distance from the measured zero-crossing sequence is selected. The symbol corresponding to this selected zero-crossing reference sequence (or the symbol sequence associated with this zero-crossing reference sequence, respectively) represents the detected symbol (the detected symbol sequence) and thus the solution of the detection problem.
From the points of view of expenditure and costs, using a zero-crossing detector is a very interesting receiver concept since it enables an (expensive) analog/digital converter to be dispensed with. The problem with this receiver concept is that the number of zero crossings in a symbol interval fluctuates in dependence on the data and other influencing variables (known system parameters, unknown interfering influences). For this reason, it is difficult to allocate successive zero crossings of the zero-crossing sequence measured directly to the equidistant symbol intervals. The advantage of conventional digital receiver concepts which have a fixed, or at least known, number of samples per symbol interval is thus not present in the receiver concept with a zero-crossing detector.
Apart from the problem of allocating zero crossings measured in the receiver to symbol intervals, a further problem is the use of zero-crossing detectors in the determination of zero-crossing reference sequences, by means of which a symbol-interval-related comparison with the measured sequence of zero crossings can be managed. An inexpensive implementation of this receiver design based on a detection of zero crossings of the received signal or of an intermediate-frequency signal is only made possible by as efficient as possible a form of calculating such zero-crossing reference sequences.